Yet far more Syrians have died in the nation's 2-year-old civil war in other ways. What's the difference?

U.S. officials and lawmakers explain that there's long-standing international agreement that chemical warfare simply isn't okay.

Democratic National Committee chairwoman Debbie Wasserman Schultz on Monday described a "100-year-old international norm not to use chemical weapons."

Have the world's nations opposed such weapons since the early 1900s?

A hundred years ago, the world had yet to face the horrors of World War I, when nearly 100,000 people were killed by grenades and artillery shells loaded with chemicals like chlorine and mustard gas. Nor had they experienced the million casualties that followed from chemical attacks worldwide.

Yet, it turns out, some international agreement against gas attacks predated the war — and even full development of the weapons themselves.

Wasserman Schultz's office sent us a declaration from world powers at an International Peace Conference at The Hague — dated 1899. It banned projectiles designed to spread "asphyxiating or deleterious gases."

It was binding only among signing countries in the case of a war between two of them, and didn't apply if a non-signing country jumped into the battle. It was worded as a limited agreement, not a moral condemnation. It came before general use of the weapons themselves.

It was signed by more than two dozen countries, and ratified by all the major powers — except the United States.

The American representative to the conference didn't agree to the declaration partly because he thought gas warfare, which had not yet been fully developed, was just as humane as other warfare, according to instructions to the American delegates and their official reports.

But the Hague Declaration marked the start of international consensus on the topic, says Richard Price, a political scientist at the University of British Columbia who wrote a book called The Chemical Weapons Taboo.

Fierce debate broke out over the reach of the ban, Price said, which restricted only projectiles, not all chemical warfare. Countries pointed fingers as thousands died in brutal chemical attacks in which victims choked and burned.

Peace treaties, then the 1925 Geneva Conference, went much further than the Hague Declaration. World leaders in Geneva noted that "the use in war of asphyxiating, poisonous or other gases, and of all analogous liquids, materials or devices, has been justly condemned by the general opinion of the civilized world."

They wrote that prohibition of such weapons would be "universally accepted as part of international law" and appealed to the "conscience" of nations.

The Geneva Protocol has since been ratified by 137 states, according to a white paper from the White House Office of Legislative Affairs provided by Wasserman Schultz's office.

But the United States, which pushed for the Geneva Protocol, didn't ratify it until 1975.

Since then, chemical attacks have been rare, with just a few notable exceptions — such as Iraq's use of them in the 1980s against Iran. Those attacks, it turned out, took place with American support.

Syria, by the way, was among countries in agreement against chemical weapons, ratifying the Geneva Protocol in 1968.

But Syria has been less cooperative since then, as one of the only holdouts to 1992's Chemical Weapons Convention. (The United States ratified that one.) That agreement to entirely eliminate chemical weapons "for the sake of all mankind" includes 189 nations that represent about 98 percent of the world's population, according to the United Nations.

It may not be binding law for Syria, but it certainly represents an international norm, Price says.

World powers did reach some international agreement against chemical weapons more than 100 years ago, though the context requires some clarification. We rate Wasserman Schultz's claim Mostly True.

Edited for print. Read the full version at PolitiFact.com.

The statement

Says there's a "100-year-old international norm not to use chemical weapons."

The Hague Declaration of 1899 took up the use of chemical weapons, but disagreement about whether their use was humane led the American representative to hold out. Even broader international condemnation came with the Geneva Protocol in 1925, but there weren't votes in the U.S. Senate to ratify that agreement until the 1970s. Now most of the world agrees such weapons ought to be eliminated entirely. But it was a long slog to get there. We rate the claim Mostly True.